Dual band wlan communication frequency synthesizer technique

ABSTRACT

A dual band WLAN (Wireless Local Area Network) communications technique is provided where a frequency synthesizer unit generates an LO (Local Oscillator) signal at a frequency between both frequency bands and two downconversion units and/or two upconversion units are provided. One of the units performs conversion between the LO signal and an IF (Intermediate Frequency) signal while the other conversion takes place between the IF signal and a zero-IF or low-IF signal. Signal processing is performed on the zero-IF or low-IF signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to WLAN (Wireless Local Area Network)communications devices and corresponding methods and in particular tothe operation of dual band WLAN communications devices that operate at afrequency in one of two different frequency bands.

2. Description of the Related Art

A wireless local area network is a flexible data communications systemimplemented as an extension to or as an alternative for, a wired LAN.Using radio frequency or infrared technology, WLAN systems transmit andreceive data over the air, minimizing the need for wired connections.Thus, WLAN systems combine data connectivity with user mobility.

Today, most WLAN systems use spread spectrum technology, a wide-bandradio frequency technique developed for use in reliable and securecommunication systems. The spread spectrum technology is designed totrade-off bandwidth efficiency for reliability, integrity and security.Two types of spread spectrum radio systems are frequently used:frequency hopping and direct sequence systems.

The standard defining and governing wireless local area networks thatoperate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allowhigher data rate transmissions, the standard was extended to 802.11bthat allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum.Further extensions exist.

Examples of these extensions are the IEEE 802.11a, 802.11b and 802.11gstandards. The 802.11a specification applies to wireless ATM(Asynchronous Transfer Mode) systems and is primarily used in accesshubs. 802.11a operates at radio frequencies between 5 GHz and 6 GHz. Ituses a modulation scheme known as Orthogonal Frequency DivisionMultiplexing (OFDM) that makes possible data speeds as high as 54 Mbps,but most commonly, communications take place at 6 Mbps, 12 Mbps, or 24Mbps. The 802.11b standard uses a modulation method known asComplementary Code Keying (CCK) which allows high data rates and is lesssusceptible to multi-path to propagation interference. The 802.11gstandard can use data rates of up to 54 Mbps in the 2.4 GHz frequencyband using OFDM. Since both 802.11g and 802.11b operate in the 2.4 GHzfrequency band, they are completely inter-operable. The 802.11g standarddefines CCK-OFDM as optional transmit mode that combines the accessmodes of 802.11a and 802.11b, and which can support transmission ratesof up to 22 Mbps.

WLAN receivers, transmitters and transceivers, as well as other datacommunications devices, usually have a system unit that processes radiofrequency (RF) signals. This unit is usually called front end.

Basically, a receiver side front end comprises RF filters, intermediatefrequency (IF) filters, multiplexers, demodulators, amplifiers and othercircuits that could provide such functions as amplification, filtering,conversion and more. Referring to FIG. 1, the front end usually includesan analog front end 100 which is the analog portion of a circuit, whichprecedes analog-to-digital conversion. Thus, the analog front end 100performs some analog signal preprocessing in unit 110 and some otherfunctions as described above, and then outputs the analog signal to ananalog-to-digital converter 130. The quantized, i.e. digitized, outputsignal of the analog-to-digital converter 130 is then supplied to adigital signal processor 140.

As can be seen from FIG. 1, the analog front end 100 of conventionaldata communications receivers may further have a unit 120 fordownconverting the received (and preprocessed) analog signal.Conventionally, RF carriers conveying data by way of some modulationtechnique are downconverted from the high frequency carrier to someother intermediate frequency through a process called mixing. Followingthe mixing process, the baseband signal is recovered through some typeof demodulation scheme.

Receiver architectures exist where unit 120 has zero-IF and/or low-IFtopology. This will now be explained in more detail with reference toFIGS. 2 and 3.

FIG. 2 is a simplified diagram illustrating the zero-IF approach forintegrated receivers. In the zero-IF approach, the incoming signal,which is at radio frequency, is converted by mixer 200 directly tobaseband (BB). Such direct conversion architectures have simplifiedfilter requirements and can be integrated in a standard silicon process,making this design potentially attractive for wireless applications.However, there may be problems with the DC offset, IQ mismatch and withlow frequency noise.

FIG. 3 illustrates the low-IF approach. As can be seen, the low-IFarchitecture operates at an intermediate frequency close to the baseband(like the zero-IF approach) and can therefore be integrated like thezero-IF circuits. However, there is a second downconverter 330 toconvert the IF signals to baseband. Low-IF devices can avoid theproblems of DC offset, IQ mismatch and low frequency noise but mayrequire additional image rejection. For this reason, an image rejectionunit 320 is added in the low-IF topology.

While FIGS. 1 to 3 have been discussed to refer to the receiver side,the transmitter side may be similarly discussed referring to FIGS. 4 to6. A transmitter side front end comprises RF filters, IF filters,multiplexers, modulators, amplifiers, and other circuits that mayprovide such functions as amplification, filtering, conversion and more.Referring to FIG. 4, the front end usually includes a digital front end400 which is the digital portion of a circuit which precedesdigital-to-analog conversion. Thus, the digital front end 400 performssome digital signal processing and then outputs the digital signal to adigital-to-analog converter 410. The converted, i.e., analog, outputsignal of the digital-to-analog converter 410 is then supplied to ananalog front end 420.

As can be seen from FIG. 4, the analog front end 420 may have a unit 430for upconverting the analog signal received from the digital to analogconverter 410. Conventionally, baseband carriers conveying data by wayof some modulation technique are upconverted from baseband to some otherintermediate frequency through a process called mixing. Following themixing process, the IF signal is further upconverted to an RF frequencyin the desired transmission frequency band and is further processed,e.g., filtered or amplified, in unit 440.

FIG. 5 is a simplified diagram illustrating the zero-IF approach forintegrated transmitters, and FIG. 6 illustrates the low-IF approach. Ascan be seen, the low-IF architecture operates at an intermediatefrequency close to the baseband (like the zero-IF approach). Further,there are two upconverters 600 and 610 to convert the baseband frequencysignals to intermediate frequency and then from intermediate frequencyto the transmission RF frequency. Moreover, an LO-feedthroughcancellation unit 620 is added in the low-IF topology. The zero-IF andlow-IF approaches shown in FIGS. 5 and 6 have the same or similarcharacteristics and problems as discussed above with reference to FIGS.2 and 3.

Another problem with communications devices that operate in a zero-IF orlow-IF approach is that the LO signal frequency for up- anddownconversion is at the center of the received/transmitted frequencybands. A VCO (Voltage Controlled Oscillator) frequency synthesizerrunning at this frequency therefore suffers from VCO pulling whichsignificantly degrades the signal quality.

A conventional LO architecture that provides a signal at an outputfrequency with reduced pulling effect is described in US 2002/0180538A1. A VCO generates a first signal having a frequency that is a fractionof the output frequency, and a frequency shifter generates a secondsignal with a frequency substantially equal to the difference betweenthe VCO frequency and the output frequency. Single-sideband mixers areused to produce output signals at the sum of the VCO frequency and theshifted frequency while suppressing an unwanted sideband at thedifference of the two frequencies.

While this technique may be suitable for reducing the pulling effect inconventional communications devices, the architecture may have somedisadvantages when being applied to dual band WLAN devices. This is inparticular because due to the increased number of component parts, thedie size and consequently the manufacturing costs are increased.Further, the conventional techniques suffer from power consumption whichis sometimes found to be a severe detriment when designing WLAN devices.

SUMMARY OF THE INVENTION

A dual band WLAN communications technique is provided that may allow forreducing manufacturing costs, improving circuit density by reducing thenumber of component parts and thus the die size, improving theefficiency and operating range, and/or reducing the power consumption.

In an embodiment, a dual band WLAN communications device is providedthat is capable of receiving and processing an input signal at afrequency in one of two different frequency bands. The dual band WLANcommunications device comprises a frequency synthesizer unit that isadapted to generate an LO signal at a frequency between a firstfrequency band of the two different frequency bands and a secondfrequency band of the two different frequency bands. The device furthercomprises a first downconversion unit that is connected to receive theinput signal and the LO signal and generate an IF signal therefrom and asecond downconversion unit that is connected to receive the IF signaland generate a zero-IF or low-IF signal therefrom. The device furthercomprises a single processing unit that is adapted to perform signalprocessing on the zero-IF or low-IF signal.

In another embodiment, there is provided a dual band WLAN communicationsdevice that is capable of transmitting an output signal at a frequencyof one of two different frequency bands. The dual band WLANcommunications device comprises a signal processing unit which isadapted to perform signal processing on a zero-IF or low-IF signal, afirst upconversion unit which is connected to receive the processedzero-IF or low-IF signal and generate an IF therefrom, and a frequencysynthesizer unit which is adapted to generate an LO signal at afrequency between a first frequency band of the two different frequencybands and a second frequency band of the two different frequency bands.The device further comprises a second upconversion unit which isconnected to receive the IF signal and the LO signal and generate theoutput signal therefrom.

According to a further embodiment, a method of operating a dual bandWLAN communications device to receive and process an input signal at afrequency of one of two different frequency bands comprises generatingan LO signal at a frequency between a first frequency band of the twodifferent frequency bands and a second frequency band of the twodifferent frequency bands, downconverting the input signal using the LOsignal to generate an IF signal, downconverting the IF signal togenerate a zero-IF or low-IF signal, and performing signal processing onthe zero-IF or low-IF signal.

In still a further embodiment, there is provided a method of operating adual band WLAN communications device to transmit an output signal at afrequency in one of two different frequency bands. The method comprisesperforming signal processing on a zero-IF or low-IF signal, upconvertingthe processed zero-IF or low-IF signal to generate an IF signal,generating an LO signal at a frequency between a first frequency band ofthe two different frequency bands and a second frequency band of the twodifferent frequency bands, and upconverting the IF signal using the LOsignal to generate the output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated into and form a part of thespecification for the purpose of explaining the principles of theinvention. The drawings are not to be construed as limiting theinvention to only the illustrated and described examples of how theinvention can be made and used. Further features and advantages willbecome apparent from the following and more particular description ofthe invention, as illustrated in the accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating the front end of a conventionaldata communications receiver;

FIG. 2 is a simplified diagram illustrating a zero-IF approach in thereceiver of FIG. 1;

FIG. 3 is a simplified diagram illustrating a low-IF approach in thereceiver of FIG. 1;

FIG. 4 is a block diagram illustrating the front end of a conventionaldata communications transmitter;

FIG. 5 is a simplified diagram illustrating a zero-IF approach in thetransmitter of FIG. 4;

FIG. 6 is a simplified diagram illustrating a low-IF approach in thetransmitter of FIG. 4;

FIG. 7 is a block diagram illustrating a dual band WLAN communicationsdevice according to an embodiment; and

FIG. 8 is a flowchart illustrating a frequency conversion processaccording to an embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The illustrative embodiments of the present invention will be describedwith reference to the figure drawings wherein like elements andstructures are indicated by like reference numbers.

Referring now to FIG. 7, a dual-band transceiver frequency conversionscheme is shown according to an embodiment. As can be seen from FIG. 7,the transceiver device has a receiver side in the lower part of thefigure and a transmitter side in the upper part. Further, there is an LOsignal generation unit 700 which may work for both the receiver andtransmitter sides.

It can further be seen from FIG. 7 that the architecture can be dividedinto an RF region, an IF region and a low-IF region. The RF region dealswith communications signals in the radio frequency range. As thecommunications device of FIG. 7 is a dual band WLAN device, the RFsignals may have frequencies in one of two different frequency bands.

The IF region deals with intermediate frequency signals at a frequencywhich does not depend on the frequency band used in the RF region.Similarly, the low-IF region performs signal processing on low-IFsignals irrespective of the frequency band of the RF signal.

As mentioned, the transceiver topology of FIG. 7 relates to a dual bandtransceiver that is capable of operating in one of two differentfrequency bands. The LO signal generation unit 700 comprises a frequencysynthesizer 705 that may have a VCO unit to generate a first LO signal.The VCO frequency is situated in between the two frequency bands.

For instance, where the two frequency bands are at about 2.4 GHz and 5.2GHz, respectively, the first LO frequency may be at about 3.6 GHz. Thus,the VCO oscillates far away from the frequency that is used fortransmission and reception. This has the advantageous consequence thatno pulling occurs.

In each of the receiver and transmitter sides, a mixer 725, 770 receivesthe first LO signal to perform downconversion or upconversion,respectively. In detail, the mixer 725 works as downconversion unit toconvert the signals from both frequency bands to one IF signal. At thetransmitter side, the mixer 770 performs upconversion on the IF signalto generate an RF signal in either one of the two frequency bands.

As may be further seen from FIG. 7, there is a second conversion stageto generate a low-IF signal from the IF signal at the receiver side, orto generate the IF signal from the low-IF at the transmitter side. Forthis purpose, additional mixers 735, 780 are provided to perform anotherdownconversion or upconversion respectively.

The mixers 735, 780 receive a second LO signal from the LO signalgeneration unit 700 to perform the conversion. As can be seen from FIG.7, the second LO signal may be generated from the VCO signal obtainedfrom the frequency synthesizer 705 by performing frequency division. Forinstance, if the first LO signal was at 3.6 GHz, the frequency divisionmay be a division by three so that the second LO signal has a frequencyof 1.2 GHz. It is noted that other embodiments exist where the frequencydivider 710 divides the frequency of the VCO signal by an integer valuedifferent from three.

Thus, there is only one VCO and PLL (Phase Locked Loop) frequencysynthesizer needed to perform the two-stage conversion of both sides.This reduces the number of transceiver blocks needed, and thus reducesthe die size and consequently the manufacturing costs. Further, thecurrent consumption is reduced, leading to an improved power design.

It is further noted that the architecture shown in FIG. 7 does notrequire dedicated units to generate LO signals for each frequency band.The only blocks dedicated to the frequency bands at transmission orreception are the amplifiers 715, 720, 760, 765 in the RF region. In anembodiment, these amplifiers may be LNA (Low Noise Amplifier) units.

As may be further seen from FIG. 7, the amplifiers 760, 765 of thetransmitter side RF region may be controlled by the level/power controlunit 790 which may also control a low pass filter 785 in the low-IFregion. Thus, the transmitter side signal processing in the low-IFregion and the frequency band specific amplification in the RF regionmay be controlled in a correlated manner, thereby increasing theoperational efficiency and overall signal quality.

Similarly, the receiver side comprises an automatic gain controller 750in the low-IF region that may provide control signals to the low passfilter 740 and the variable gain amplifier 745 in the low-IF region, aswell as a control signal to an amplifier 715 in the RF region. Thislikewise allows for controlling RF signal amplification and low-IFsignal processing in a correlated manner. While FIG. 7 shows that onlyone of the receiver side amplifiers is controlled by the automatic gaincontroller 750, other embodiments exist where both amplifiers or theamplifier of the other frequency band is controlled. Further,embodiments exist where the amplifier control is temporarily orpersistently disabled.

Similarly, while amplifiers 760 and 765 are both shown to be controlledby the level/power control unit 790, other embodiments may control onlyone of the amplifiers, or may have even this control disabled.

Referring to the IF region shown in FIG. 7, an additional amplifier 730,775 may be located between both downconverting mixers 725, 735 or bothupconverting mixers 770, 780 respectively. These amplifiers 730, 775operate at the IF signal frequency and may be suitable for adjusting thepower level in an appropriate manner, perform impedance matching, and/orserve for decoupling the respective mixers.

While the embodiment of FIG. 7 has been discussed to perform signalprocessing in the low-IF range, other embodiments exist where a zero-IFapproach is used instead or in addition to the low-IF approach.

Further, while the embodiment of FIG. 7 relates to a transceiver device,that has both receiver and transmitter capabilities, other embodimentsmay relate to dedicated dual band WLAN receivers or transmittersrespectively.

In the embodiments discussed above, communications are performed inaccordance with the IEEE 802.11a and 802.11g specifications. It ishowever noted that other embodiments may make use of two frequency bandsin accordance with other WLAN techniques.

Referring now to FIG. 8, a flowchart is shown illustrating a frequencyconversion process according to an embodiment. In step 800, an RF inputsignal is received at one of the amplifiers 715, 720. A VCO signalgenerated in a frequency synthesizer 705 of the LO signal generationunit 700 is then applied to the received input signal by means of mixer725 to generate an IF signal in step 810. the VCO signal is then used togenerate a second LO signal by performing frequency division in unit710. The frequency divided signal is then applied to the (amplified) IFsignal by means of mixer 735 to generate a zero-IF or low-IF signal instep 830. This signal is then subjected to further signal processingsuch as low-pass filtering, amplification and analog-to-digitalconversion. A similar process may take place at the transmitter side ofthe dual band communication device according to the embodiment.

As apparent from the foregoing description of the various embodiments, adual band WLAN communications technique is provided that applies atwo-stage downconversion and/or upconversion using only one frequencysynthesizer. Thus, only one VCO unit is needed for operating bothconversion stages and dealing with both the transmitter and receiverside. The VCO signal is chosen to be at an inter-band frequency therebyavoiding the signal degradation due to VCO pulling. In an embodiment,the VCO frequency, i.e., the first LO signal frequency, is chosen to benear the center frequency between both frequency bands. It is howevernoted that there may be no strict requirement to choose the VCOfrequency to be exactly at the center.

The second LO signal frequency is generated in the embodiments byperforming frequency division on the VCO signal. This again reduces thenumber of component parts and thus the power consumption.

In the embodiments described above, the two-stage dual band WLANconversion scheme of the embodiments may be implemented in the analogfront end of the communications device, thereby avoiding the need toperform complex digital signal processing.

While the invention has been described with respect to the physicalembodiments constructed in accordance therewith, it will be apparent tothose skilled in the art that various modifications, variations andimprovements of the present invention may be made in the light of theabove teachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention. Inaddition, those areas in which it is believed that those of ordinaryskill in the art are familiar, have not been described herein in orderto not unnecessarily obscure the invention described herein.

Accordingly, it is to be understood that the invention is not to belimited by the specific illustrative embodiments, but only by the scopeof the appended claims.

1-13. (canceled)
 14. A dual band WLAN (Wireless Local Area Network)communications device capable of transmitting an output signal at afrequency in one of two different frequency bands, comprising: a signalprocessing unit adapted to perform signal processing on a zero-IF orlow-IF signal; a first upconversion unit connected to receive theprocessed zero-IF or low-IF signal and generate an IF (IntermediateFrequency) signal therefrom; a frequency synthesizer unit adapted togenerate an LO (Local Oscillator) signal at a frequency between a firstfrequency band of said two different frequency bands and a secondfrequency band of said two different frequency bands; and a secondupconversion unit connected to receive said IF signal and said LO signaland generate said output signal therefrom.
 15. The dual band WLANcommunications device of claim 14, wherein said frequency synthesizerunit is adapted to generate said LO signal at a predefined frequencywhich is independent of whether the frequency of said output signal isin the first or second frequency band.
 16. The dual band WLANcommunications device of claim 15, wherein said LO signal is generatedfrom a VCO (Voltage Controlled Oscillator) signal.
 17. The dual bandWLAN communications device of claim 14, further comprising: a frequencydivider unit connected to said frequency synthesizer unit to generate asecond LO signal by dividing the frequency of said LO signal by apredefined value, wherein said first upconversion unit is connected tosaid frequency divider unit to receive said second LO signal.
 18. Thedual band WLAN communications device of claim 17, wherein saidpredefined value is an integer value.
 19. The dual band WLANcommunications device of claim 18, wherein said integer value is three.20. The dual band WLAN communications device of claim 14, capable ofreceiving an input signal at the frequency of said output signal, andfurther comprising: a first downconversion unit connected to receivesaid input signal and said LO signal and generate a reception LF signaltherefrom; and a second downconversion unit connected to receive saidreception IF signal and generate a reception zero-IF or low-IF signaltherefrom.
 21. The dual band WLAN communications device of claim 20,further comprising: a frequency divider unit connected to said frequencysynthesizer unit to generate a second LO signal by dividing thefrequency of said LO signal by a predefined value, wherein said firstupconversion unit and said second downconversion unit are connected tosaid frequency divider unit to receive said second LO signal.
 22. Thedual band WLAN communications device of claim 14, further comprising afirst amplifier and a second amplifier, the first amplifier beingadapted to operate in said first frequency band, the second amplifierbeing adapted to operate in said second frequency band, wherein at leastone of said first and second amplifiers is connected to be controlled bya power control unit.
 23. The dual band WLAN communications device ofclaim 22, wherein said power control unit is further connected to saidsignal processing unit to control the signal processing performed bysaid signal processing unit.
 24. The dual band WLAN communicationsdevice of claim 22, capable of receiving an input signal at thefrequency of said output signal, and further comprising: a thirdamplifier and a fourth amplifier, the third amplifier being adapted tooperate in said first frequency band, the fourth amplifier being adaptedto operate in said second frequency band, wherein at least one of saidthird and fourth amplifiers are connected to said signal processing unitto be controlled by an AGC (Automatic Gain Control) unit of said signalprocessing unit. 25-39. (canceled)
 40. A method of operating a dual bandWLAN (Wireless Local Area Network) communications device to transmit anoutput signal at a frequency in one of two different frequency bands,comprising: performing signal processing on a zero-IF or low-IF signal;upconverting the processed zero-IF or low-IF signal to generate an IF(Intermediate Frequency) signal; generating an LO (Local Oscillator)signal at a frequency between a first frequency band of said twodifferent frequency bands and a second frequency band of said twodifferent frequency bands; and upconverting said IF signal using said LOsignal to generate said output signal.
 41. The method of claim 40,wherein said LO signal is generated at a predefined frequency which isindependent of whether the frequency of said output signal is in thefirst or second frequency band.
 42. The method of claim 41, wherein saidLO signal is generated from a VCO (Voltage Controlled Oscillator)signal.
 43. The method of claim 40, further comprising: generating asecond LO signal by dividing the frequency of said LO signal by apredefined value, wherein said zero-IF or low-IF signal upconversionuses said second LO signal.
 44. The method of claim 43, wherein saidpredefined value is an integer value.
 45. The method of claim 44,wherein said integer value is three.
 46. The method of claim 40, adaptedto receive an input signal at the frequency of said output signal, andfurther comprising: downconverting said input signal using said LOsignal to generate a reception IF signal; and downconverting saidreception IF signal to generate a reception zero-IF or lowIF signal. 47.The method of claim 46, further comprising: generating a second LOsignal by dividing the frequency of said LO signal by a predefinedvalue, wherein said zero-IF or low-IF signal upconversion and saidreception IF signal downconversion use said second LO signal.
 48. Themethod of claim 40, further comprising operating a first amplifier insaid first frequency band and a second amplifier in said secondfrequency band, wherein at least one of said first and second amplifiersis controlled by a power control unit.
 49. The method of claim 48,wherein said signal processing is controlled by said power control unit.50. The method of claim 48, adapted to receive an input signal at thefrequency of said output signal, and further comprising: operating athird amplifier in said first frequency band and a fourth amplifier insaid second frequency band, wherein at least one of said third andfourth amplifiers is controlled by an AGC (Automatic Gain Control) unit.51. The method of claim 40, wherein said LO signal generation and saidupconversion steps are performed by an analog front end of a dual bandWLAN communications device, and said signal processing comprisesperforming digital-to-analog signal conversion.
 52. The method of claim40, adapted to be performed in accordance with the IEEE 802.11a and IEEE802.11g specifications.